This invention pertains to powder coating and adhesive formulations utilizing organosilane compounds, or their hydrolyzates or condensates, as crosslinkers and/or adhesion promoters.
Powder coatings are an environmentally friendly system, making them a desirable alternative to conventional solvent borne coating systems. Overall, operating and material costs are such that powder coating competes favorably with the solvent and waterborne markets.
Although powder coating represent only a portion of the total market, as compared to solvent type systems, the technology has considerable advantages. Most notably, since powder coating lacks solvent, there are no VOC issues with which to be concerned. Further, since there is less waste generated, the powder coating technique has less impact on the environment.
Powder coating involves several steps, the most critical of which is the premixing of the ingredients. During this initial phase, the binder, together with the other additives are mixed thoroughly in the equipment. Inadequate premix in the first stage could lead to a non-homogenous composition of the coating and poor mechanical properties or surface defects in the final product. The resulting premix is then fed into an extruder. The molten material produced from the extruder is cooled and squeezed into easily breakable strips. The strips are then ready for grinding to a particular particle size range.
The most common method for the application of powder coatings is by electrostatic spraying. The basic principle of this method involves propelling the powder, via compressed air, through a spray gun where it becomes electrostatically charged. In addition to charging the powder the gun also serves to deposit the powder supplied by the feeder. When the electric field is removed, the charged particles are still held on the surface, attracted by charges on the substrate. The uncharged powder in the overspray is collected and reused.
Another commonly used method of powder coating is triboelectric spraying. This is similar to electrostatic spraying, except the particles are positively charged (electrostatically charged particles have a negative charge). A new technique, that is being developed for flat surfaces, employs electromagnetic brush technology which enables efficient high speed application of very thin layers with no recycling.
Among the drawbacks of thermoset powder coating systems is the difficulty in making tough films from ingredients that are low in molecular weight and therefore able to flow easily under shear conditions. Since the application of the powder coating involves overspraying, specialized recovery equipment is also necessary to reclaim the unused powder. Substrates must also be able to withstand the powder coating cure temperatures which typically range between 150 and 190xc2x0 C.
The silanes of the present invention advantageously can be used to improve the physical and chemical properties of thermoplastic powder coatings. These powder coating formulations do not require curing agents and can be applied via the aforementioned electrostatic or tribostatic spray techniques. However, most thermoplastic powder is applied by passing the heated substrate through a fluidized bed.
Silanes are known to be useful in liquid coatings. For example in WO 96/39468 sprayable liquid coating compositions are described which include a film-forming reactive silyl group-containing compound and polymer microparticles insoluble in the liquid coating composition.
One aspect of the invention is a powder coating formulation which includes as a component thereof a silane of the formula: 
where R1 is a hydrocarbon, acyl, alkylsilyl or alkoxysilyl group; R2 is a monovalent hydrocarbon group; R3 is alkylene, optionally interrupted with one or more ether oxygen atoms; a is 0 or 1; Z is a direct bond or a divalent organic linking group; X is an m-valent organic group or H; and m is 1 to 20.
Silanes, useful in the present invention, can be prepared via many synthetic routes. For instance, a terminally unsaturated organic compound can be hydrosilylated to give a silane. Further chemical reactions can be carried out on silanes having organo functional groups. Silyl carbamates, useful in the invention can be prepared by reaction of polyol compounds with isocyanatopolyalkoxysilanes. Other carbamates, useful in the invention can be prepared by reaction of polyisocyanates with hydroxyalkylpolyalkoxy silanes. Such silyl carbamates can also be made by reaction of a polyisocyanate with a terminally unsaturated alcohol, and subsequent hydrosilation. Silyl isocyanates can be oligomerized to: uretdiones, allophanates, biurets, isocyanurates. Silyl amines can be reacted with anhydrides to yield amides or with isocyanates to yield ureas.
Powder coatings within the invention may be based on a conventional resin system, in such coatings the silane component of formula I function as property modifying and/or crosslinking additives. Silane compounds of formula I, where m is greater than 1, and especially those in which X is polymeric, may be used as the base crosslinking resin for the coating formulation.
Powder coating systems may be based on a number of thermosetting chemistries. Well known powder coating systems include polyurethane systems based on blocked polyisocyanates and polyols, especially polyester or poly(meth)acrylate polyols; acid functional acrylic or other acid functional polymers cured with epoxy functional curing agents; anhydride/epoxy systems; epoxy/polyol systems; hybrid systems employing epoxide resins and polyesters with both carboxyl and hydroxyl functionality; systems based on hydroxyalkylamides and acid functional polymers. Examples of suitable epoxy resins include bisphenol A-type polyepoxides, glycidyl methacrylate copolymers and epoxy-novolac resins. Typically the systems are designed to melt-flow and to cure concurrently in a single heating step, although in some cases a UV curing system may be employed to separate the film-forming melt flow stage from the curing stage.
Particular powder coating systems in which the inventive compounds are useful include polyester-urethane powder coatings in which hydroxyl-functional polyester resins are cured with polyisocyanates. The polyisocyanates are blocked internally or blocked with a blocking agent. The leading blocking agent is xcex5-caprolactam. When the powder-coated part is heated, xcex5-caprolactam, is volatilized, unblocking the isocyanates groups and leaving them free to react with the hydroxyl functionality on the polyester resin. The most common blocked isocyanate is the caprolactam-blocked IPDI (isophoronediisocyanate) such as Hxc3xcls Vestagon B1530. The polyisocyanates can be blocked internally by a dimerization process to form uretidone. One such uretidone compound that is commercially available is Hxc3xcls Vestagon BF 1540 (an IPDI uretidone).
Typical hydroxyl-functional polyester resins used in polyester-polyurethane systems are derived from condensation polymerization of glycol, dicarboxylic acid and polyol (more than two hydroxyl groups in the monomer). Frequently used glycols are trimethylpentanediol and neopentyl glycol; polyols include trimethylolpropane and trimethylolethane. Dibasic acids include isophthalic acid and terephthalic acid. A standard method of preparation of hydroxyl-functional polyester resin for polyurethane powder coating with reaction details is given in the book of Oldering and Hayward [Oldering, P., and Hayward, G., in Resins for Surface Coatings, Vol. II, SITA Technology, London, 1987, p.137.]. Different patents describe methods for making hydroxyl functional polyester resin for polyurethane powder coatings with minor variations with respect to the acid value, hydroxyl value, functionality of the resin and the choice of the raw materials. The hydroxyl-functional polyester for powder coatings could have an acid number less than 10, a molecular weight of 2800-3200, a hydroxyl number of 84, and a softening point at 95-100xc2x0 C.
A typical IPDI-crosslinked powder coating formulation is as follows: 52-53% hydroxyl polyester, 12-13% IPDI crosslinker, 34% pigment filler 1% flow modifier and small amount of other ingredients.
Other powder coating systems in which the inventive silane compounds are useful are acrylic-urethane powder coatings. In such systems hydroxyl-functional acrylic resins are used to prepare acrylic-urethane powders in essentially analogous way to polyester-urethanes. The hydroxyl-functional acrylic resins can be prepared as a copolymer of methyl acrylate, styrene, acrylate esters, hydroxyethyl methacrylate and acrylic acid. Typically, about 9-10% of hydroxylethylmethacrylate is needed to yield a resin with a hydroxyl value of 40, and about 2% of acrylic acid to result in acid value of 16. Methyl methacrylate and styrene copolymers yield high Tg (e.g. 95xc2x0 C.-105xc2x0 C.) and are usually flexibilized with longer chain acrylates or methacrylates such as butyl, ethyl, or 2-ethylhexyl. Butyl acrylate often is preferred as an acrylic comonomer because its excellent UV resistance combined with high flexibility. The molecular weight of acrylic copolymer resins are usually 5,000-20,000.
Formulations for acrylic-urethane powder coatings are similar to the polyester-urethane formulations except for the use of the acrylic resin in place of the polyester resin.
Still other powder coating formulations in which the silane compounds of the invention may be employed are based on glycidyl functional acrylic resins (especially glycidyl methacrylate copolymers) and compounds having two or more carboxylic acid groups such as dodecanedioic acid. Exemplary glycidyl methacrylate copolymers are made from 15-35 weight percent glycidyl methacrylate, 5-15 weight percent butyl methacrylate and the balance styrene and/or methyl methacrylate; have a number average molecular weight of below 2,500; have a Tg of above 80xc2x0 C.; and have a melt viscosity of less than 400 poise (40 Pascal seconds) at 150xc2x0 C.
The inventive silane compounds also may be employed in TGIC/polyester powder coating systems. In such systems carboxyl-functional polyester resins are cured with TGIC (triglycidyl isocyanurate). A polyester resin suitable for production of powder coatings with TGIC in the formulation is described in the patent of DSM resins BV [DSM Resins BV, Belg. Pat 898 099, 1982]. The resin is obtained by fusion esterification of neopentyl glycol, 1,4-cyclohexanedimethanol, 1,6-hexane diol, trimethylolpropane, terephthalic acid and adipic acid. Products with average molecular weight between 4,500 and 12,500 with acid values of 10-26 mg KOH/g and Tg of 40-85xc2x0 C. are obtained which are suitable for making powder coatings containing 1.4-5.3% by weight of TGIC. The silanes of the present invention can also be used with similar carboxyl functional polyesters that are cured with hydroxyalkylamides.
The powder coating formulations of the invention may also be based on a thermoplastic polymer such as nylon, polyolefins (for instance polypropylene and polyethylene), polyphenylene sulfide or polyvinylchloride.
The softening points of the base components of the powder coating compositions are such that the necessary additives to produce the coating formulations of the invention can be worked in at temperatures between about 80xc2x0 C. and 140xc2x0 C. and yet produce a composition which can be subsequently extruded and ground to a fine free-flowing powder of about 20-120 micrometer size. Solid additives that melt and are compatible with the formulation are preferred. However, compatible liquids can be utilized via a master-batch or on an inert carrier.
A necessary component in the compositions of the present invention is a polyalkoxy or polyacyloxy silane compound, of formula I: 
where R1 is a hydrocarbon, acyl, alkylsilyl or alkoxysilyl group; R2 is a monovalent hydrocarbon group; R3 is alkylene, optionally interrupted with one or more ether oxygen atoms; a is 0 or 1; Z is a direct bond or a divalent organic linking group; X is an m-valent organic group; and m is 1 to 20. The number m is preferably 2 to 6, most preferably 2.
The group R1 may be an alkyl, aryl, alkaryl, aralkyl or acyl group, for instance, methyl, ethyl, n-propyl, isopropyl, butyl, t-butyl, phenyl, benzyl, tolyl, benzoyl, or acetyl. R1 may also be an alkylsilyl group, for instance a trialkylsilyl group such as trimethylsilyl, triethylsilyl or tripropylsilyl or an arylalkyldialkylsilyl group such as benzyldimethylsilyl or tolyldimethylsilyl. R1 may also be an alkoxysilyl group, for instance a trialkoxysilyl group such as trimethoxysilyl, triethoxysilyl and tripropoxysilyl; an alkyldialkoxysilyl such as methyldimethoxysilyl, methyldiethoxysilyl, ethyldimethoxysilyl, ethyldiethoxysilyl, methyldipropoxysilyl and ethyldipropoxysilyl; or dialkylalkoxysilyl such as dimethylmethoxysilyl, dimethylethoxysilyl, dimethylpropoxysilyl, diethylmethoxysilyl, diethylethoxysilyl, and diethylpropoxysilyl. Preferably R1 is a linear, branched or cyclic C1-C6 alkyl group or an acetyl group. Most preferably R1 is ethyl or methyl. Suitable R2 hydrocarbon groups are aryl, alkenyl or alkyl groups, which may be linear, branched or cyclic, especially lower (C1-C4)alkyl groups such as methyl or ethyl. R3 is suitably C2-C12 linear, branched or cyclic alkylene, preferably C2-C6 alkylene. Exemplary R3 groups are propylene, ethylcyclohexylene, 3,3-dimethylbutylene, ethylene and methylene.
Z is a direct bond or a divalent organic linking group. Suitable divalent linking groups include ester, amide, urea, uretdione, carbamate, carbonate, aromatic ring, heterocyclic ring, allophanate, biuret, amine, ether and thioether. The Z groups may be the same or different.
The m-valent organic group X is typically a residue of an organic compound having one or more carboxylic acid, halide, alcohol, isocyanate, amine, epoxy, thiol, or other pendant or terminal functional groups which have been reacted in a known manner to form the linkages Z. The residue X may be polymeric, for instance polyacrylate, polycarbonate, polyurethane, polyalkylene, polyester, polyamide, polyether, and combinations of these. Preferably the group X is an aliphatic, cycloaliphatic or aromatic hydrocarbon group, preferably a C4-C24 hydrocarbon group, especially a saturated linear, branched or cyclic aliphatic hydrocarbon group. Exemplary X groups include 2,3-butylene; 1,6-hexylene; 1,4-cyclohexanedimethylene; 1,4-cyclohexylene; 1,7-heptylene; 1,8-octylene; 1,12-dodecylene; 1,10-decylene; 1,9-nonylene; 4,4xe2x80x2-isopropylidenediphenylene; 4,4xe2x80x2-isopropylidenedicyclohexylene; 1,4-butylene; phenylene; methylphenylene, 1,3-(xcex1,xcex1,xcex1xe2x80x2,xcex1xe2x80x2-tetramethyl)xylylene.
Preferred silanes useful in the inventive formulations may be characterized by formula II: 
where R1, R2, R3, a, m and X are as previously defined, and A and B are independently xe2x80x94NHxe2x80x94 or xe2x80x94Oxe2x80x94.
Silyl carbamates of formula II, where A=NH and B=O, may be prepared by reaction of polyol compounds with isocyanatoalkylalkoxysilanes. Novel silane carbamates which may be obtained in this manner include carbamates so formed in which the polyol compound is a hydrocarbon diol. Linearly symmetrical diols such as 1,4-cyclohexanediol, 4,4xe2x80x2-isopropylidenedicyclohexanol and 1,4-cyclohexanedimethanol also produce preferred silane carbamate compounds of the invention. Silyl carbamates of this type can also be made by the reaction of a silyl isocyanate with a polymeric polyol, such as a polyether polyol, a polyester polyol, a polybutadiene polyol or a polyacrylate polyol.
Examples of suitable isocyanatoalkylalkoxysilanes are isocyanatopropyltrimethoxysilane, isocyanatopropylmethyldimethoxysilane, isocyanatopropylmethyldiethoxysilane, isocyanatopropyltriethoxysilane, isocyanatopropyltriisopropoxysilane, isocyanatopropylmethydiisopropoxysilane; isocyanatoneohexyltrimethoxysilane, isocyanatoneohexyldimethoxysilane, isocyanatoneohexydiethoxysilane, isocyanatoneohexyltriethoxysilane, isocyanatoneohexytriisopropoxysilane, isocyanatoneohexyldiisopropoxysilane, isocyanatoisoamyltrimethoxysilane, isocyanatoisoamyldimethoxysilane, isocyanatoisoamylmethyldiethoxysilane, isocyanatoisoamyltriethoxysilane, isocyanatoisoamyltriisopropoxysilane, and isocyanatoisoamylmethyldiisopropoxysilane.
Examples of suitable polyol compounds which will produce solid silyl carbamates with isocyanatoalkylalkoxysilanes include 2,3-butanediol; 1,6-hexanediol; 1,4-cyclohexanedimethanol; 1,4-cyclohexanediol; 1,7-heptanediol; 1,8-octanediol; pentaerythritol; 1,12-dodecanediol; 1,10-decanediol; 3,6-dimethyl-4-octyne-3,6-diol; 1,9-nonanediol; bisphenol A; hydrogenated bisphenol A (i.e., 4,4xe2x80x2-isopropylidenedicyclohexanol); and 1,4-butanediol.
The reaction product of polyol and isocyanatopropyltrialkoxysilane can be a liquid of various viscosities or a solid at room temperature.
The isocyanatopropylalkoxysilane preferably has a high purity, i.e. above about 95%, and is preferably free from impurities and/or additives, such as transesterification catalysts, which can promote side reactions. Examples of undesirable transesterification catalysts are acids, bases and organometallic compounds. For isocyanatopropyltrimethoxysilane a purity of at least 98% is preferred. This may be accomplished by distilling commercially available isocyanatopropyltrimethoxysilane, available as SILQUEST(copyright) Y-5187 silane from Witco Corporation, to remove impurities such as (3-trimethoxysilylpropyl)methylcarbamate and others as well as inhibitors, catalysts and other additives.
The preferred glycols are the symmetrical ones such as 1,4-butanediol, 1,4-cyclohexanediol, and 1,4-cyclohexanedimethanol. While the reaction product of 1,4-butanediol and isocyanatopropyltriethoxysilane is a solid at room temperature, the reaction product of either 1,2-butanediol or 1,3-butanediol with isocyanatopropyltriethoxysilane is a liquid at room temperature. Similarly, the reaction product of 1,4-cyclohexanediol and isocyanatopropyltriethoxysilane is a solid at room temperature but the reaction product of 1,2-cyclohexanediol and isocyanatopropyltriethoxysilane is a liquid at room temperature.
An illustrative reaction to produce silane carbamate compounds from a diol is depicted below: 
where R3 and X are as previously defined. The reaction is catalyzed by a tin catalyst such as dibutyltin dilaurate (DBTDL); dibutyltin oxide; dibutyltin dichloride; dibutyltin diacetate; dibutyltin dimaleate; dibutyltin dioctoate; dibutyltin bis(2-ethylhexanoate); tin acetate; tin octoate; tin ethylhexanoate; tin laurate. Other urethane catalysts include: K-KAT(copyright)s (zirconium, aluminum, or bismuth compounds); diazabicyclo[2,2,2]octane (DABCO); N,N-dimethylcyclohexylamine (DMCA); 1,8-diazabicyclo[5,4,0]-undec-7-ene (DBU); 1,5-diazabicyclo[2,3,0]non-5-ene (DBN). The reactions are typically exothermic and the temperature should be controlled to minimize the color in the final product. Excessive exotherm may also introduce impurities by way of side reaction. It is recommended that the exotherm be controlled so as not to allow the temperature of the reaction mixture to exceed 150xc2x0 C., more preferably no more than about 110xc2x0 C.
In a similar way triols, tetrols, pentols and hexols may be reacted with equivalent amounts of isocyanatoalkyltriethoxysilane. Such materials include gylcerine, trimethylolpropane, trimethylolethane, pentaerythritol, inositol, mannitol, sorbitol, fructose, fucose and glucose.
A preferred silane of the invention is the adduct compound prepared from two moles of isocyanatopropyltriethoxysilane and one mole of 1,4-cyclohexanedimethanol.
Examples of these carbamates include bis[3-(triethoxysilyl)propyl]-1,4-cyclohexanedimethyldicarbamate, bis[3-(trimethoxysilyl)propyl]-1,4-cyclohexanedimethyldicarbamate; bis[3-(methyldimethoxysilyl)propyl]-1,4-cyclohexanedimethyldicarbamate; bis[3-triethoxysilyl)propyl]-1,2-cyclohexanedimethyldicarbamate; bis[3-methyldiethoxysilyl)propyl]-1,2-cyclohexanedimethyldicarbamate; bis[3-triethoxysilyl)propyl]-1,4-butanedicarbamate, bis[3-(methyldiethoxysilyl)propyl]-1,4-butanedicarbamate; bis[3-triethoxysilyl)propyl]-2,3-butanedicarbamate; bis[3-(triethoxysilyl)propyl]-1,10-decanedicarbamate; bis[3-(trimethoxysilyl)propyl]-1,10-decanedicarbamate, bis[3-trimethoxysilyl)propyl]-1,6-hexanedicarbamate, tris[3-trimethoxysilyl)propyl]-1,2,3-propanetricarbamate, tris[3-triethoxysilyl)propyl]-1,2,3-propanetricarbamate, tris[3-(methyldimethoxysilyl)propyl]-1,2,3-propanetricarbamate.
Other silyl carbamates of formula II, where A=O and B=NH, may be prepared by reaction of an alkoxysilylalcohol with a polyisocyanate. Such silyl carbamates may also be made by reaction of a polyisocyanate with a terminally unsaturated alcohol, and subsequent hydrosilation.
An illustrative reaction to produce a silyl carbamate from a diisocyanate and an alkoxysilylalcohol is depicted below: 
where R3 and X are as previonsly defined. This reaction may be catalyzed by the aforementioned catalysts and the same recommendations apply. Preferred compounds X(Nxe2x95x90Cxe2x95x90O)n are polyisocyanate prepolymers derived from diisocyanates and polyols. In a similar way triisocyanates, such as IPDI isocyanurate and HDI isocyanurate, may be reacted with equivalent amounts of alkoxsilyl alcohol. An alkoxysilyl polyol may be reacted with an equivalent amount of polyisocyanate in this reaction.
Examples of suitable alkoxysilyl alcohols include N(3-methyldiethoxysilylpropyl)-2-hydroxy-1-propyl carbamate;. N(3-methyldiethoxysilylpropyl)-1-hydroxy-1-propyl carbamate; N(3-triethoxysilylpropyl)-2-hydroxy-1-propyl carbamate; N(3-triethoxysilylpropyl)-1-hydroxy-1-propyl carbamate; N-(3-triethoxysilylpropyl)-4-hydroxybutyramide. Other examples are described in U.S. Pat. No. 5,587,502, the teachings of which are incorporated herein by reference. Suitable polyisocanates include, but are not limited to: 1,6-hexane diisocyanate (HDI); isophorone diisocyanate (IPDI); 2,4-toluene diisocyanate; 2,6-toluene diisocyanate; mixtures of 2,4-toluene diisocyanate and 2,6-toluene diisocyanate (TDI); diphenylmethane 4,4-diisocyanate (MDI); bis(4-isocyanatocyclohexyl)methane (H12MDI); 1,3-(xcex1,xcex1,xcex1xe2x80x2,xcex1xe2x80x2-tetramethyl)xylene diisocyanate (TXMDI); xcex1,xcex1-dimethylmetaisopropylbenzyl isocyanate (m-TMI); and the dimers, trimers, biurets, allophanates, and other oligomers of such polyisocyanates. The polyisocyanate may also be a polymeric polyurethane xe2x80x9cprepolymerxe2x80x9d, such as those derived from the reaction of the forementioned polyisocyanates and a polyether polyol, a polyester polyol, a polybutadiene polyol or a polyacrylate polyol.
Alternatively carbamates useful in the present invention may be prepared by reaction of a terminally unsaturated alcohol with a polyisocyanate, and hydrosilation of the terminally unsaturated polyurethane intermediate in the presence of a suitable catalyst. An example of this reaction sequence, using allyl alcohol and a platinum catalyst, is depicted below: 
where R1 and R2 are as previously defied, X is an organic residue group and n is 1 to 10. For example the isocyanate OCNxe2x80x94Xxe2x80x94(NCO)n can be HDI, IPDI, H12MDI, TDI, MDI, TMXDI, TMI, or a dimer, trimer, allophanate or oligomeric prepolymer thereof. A terminally unsaturated polymeric alcohol can also be used in above sequence in place of allyl alcohol. Similarly, a terminally unsaturated isocyanate can be reacted with a polyol to give a terminally unsaturated carbamate, which can be hydrosilated in lie manner: 
Silane urea compounds may also be employed in the formulations of the invention. Such compounds correspond to formula II where both A and B are NH. They are suitably prepared by reaction of an alkoxysilylalkylamine with an organic isocyanate or by reaction of an organic amine with an alkoxysilylalkylisocyanate. The reaction is analogous to the reaction of isocyanate and alcohol but typically does not require a catalyst.
Examples of alkoxysilylalkylamines include 3-aminopropyltriethoxysilane (SILQUEST(copyright) A-1100); 3-aminopropyltrimethoxysilane (SILQUEST(copyright) A-1110); 3-phenylaminopropyltrimethoxysilane (SILQUEST(copyright) Y-9669); N,N-bis(3-propyltrimethoxysilyl)amine (SILQUEST(copyright) A-1170); 3-aminopropyl(methyldiethoxysilane); 3-aminopropyl(methyldimethoxysilane); aminoneohexyltriethoxysilane (SILQUEST(copyright) Y-11637).
Examples of organic (poly)amines include: n-octylamine, n-hexylamine, ethylenediamine; propylenediamine; 1,4-diaminobutane; 1,6-diaminohexane; IPDA; TDA; MDA; H12MDA. The organic amine may be polymeric, such as the JEFFAMINES(copyright) (polyether polyamines).
In all of the foregoing descriptions of synthetic methods it will be understood that if trialkoxysilane starting compounds are replaced by corresponding dialkoxyalkylsilanes, triacyloxysilanes or diacyloxyalkylsilanes, other compounds of formula I will be produced.
Other conventional additives additionally may be employed.
Examples of conventional powder coating additives which may be incorporated into the coating formations of the invention include accelerating catalysts, pigments, leveling agents, flow modifiers, light stabilizers, antioxidants, and fillers, all well known in the art. These ingredients may be employed in the inventive compositions in conventional amounts. The catalysts, typically titanate or tin compounds, are conventionally added in amounts of 0.05%-1.5% by weight, preferably 0.1%-0.5% by weight. The fillers and colorants may be added in large amounts, often as much as 50% or more, and sometimes 60% or more of the total composition weight. Clearcoat formulations may be entirely free of fillers and pigments.
The silane compounds as described above are useful as crosslinkers, as adhesion promoters, and/or as modifiers of such film properties as environmental resistance or mar resistance. At the elevated temperatures used to cure the powder coating, the carbamate groups of the molecules, if present, can react with isocyanate groups to form allophanates and biurets and/or the SiOR1 groups can react with polyols in the coating in exchange reactions.
Silanes of formula I may be employed alone or in combination with conventional crosslinking agents. They may be used to furnish the principal crosslinking network, or they may reinforce conventional crosslinking networks. In compounds where the group X is polymeric and the compound is a meltable solid, the silane of formula I may be employed as the base resin of the coating system.
In addition to the methods described above, polymeric silanes of the present invention can be synthesized by copolymerizing a silane monomer with a non-silane monomer. For example 3-methacryloxypropyltrimethoxysilane, SILQUEST(copyright)A-174, can be added to an alkyl(meth)acrylate monomer such as methyl methacrylate (and/or substituted (meth)acrylates; such as hydroxyethyl acrylate, hydroxyethyl methacrylate, glycidyl acrylate or glycidyl methacrylate) and copolymerized to give a siloxy functional poly(meth)acrylate (or if glycidyl methacrylate was included, a siloxy functionalized GMA resin).
Preferred silane compounds used in the formulations of the invention are meltable solids which make them especially suited for incorporation into conventional powder coating formulations without substantially altering the melt properties of the material. Melting points desirably will fall within the range of from about 30xc2x0 C. to about 170xc2x0 C., preferably from about 40xc2x0 C. to about 120xc2x0 C. and more preferably from about 50xc2x0 C. to about 110xc2x0 C.
The molecular weight of the silane compounds of the invention is preferably about 8000 Daltons or less, more preferably about 5000 Daltons or less.
Although solids are a preferred embodiment of the present invention, silanes in the form of liquids or waxes can be used as additives in conventional powder coating resin systems.
A preferred mode of adding silanes that are not solids is via an inert carrier, such as silica, carbon black or porous polymers. Liquids may also be added in the form of solid solutions, as a xe2x80x9cmaster-batch,xe2x80x9d or encapsulated. Silanes of the present invention may be added to a powder coating formulation via incorporation in/on pigments or fillers. Titanium dioxide is a preferred carrier for these silanes. Silanes in these forms may be added to the powder coating formulation in the same manner as solid silanes.
In a typical powder coating formulation, a useful quantity of the silane additive will be in the range of from about 0.5 to about 30% by weight of the formulation, more preferably from about 2% to about 10% by weight. In some formulations, however, higher or lower amounts may prove beneficial. Further, as noted above, silane compounds of formula I in which X is polymeric may be used as the base resin. Therefore the inventive powder coating formulations should not be considered to be restricted to these quantity ranges.
Powder adhesives may be similarly formulated. Such adhesives may be applied to one or both of the substrates to be joined in the same manner as a powder coating, for instance by electrostatic spraying, triboelectric spraying, electromagnetic brushing or via fluidized bed. The substrates are joined with heating to fuse the coating, and, if curable, allowed to cure. Upon cooling a bonded assembly is formed. Typically the size distribution of such powder adhesives is such that at least 50% by weight will pass through a 200 mesh screen.